A common method of perforating oil and gas wells for the production of petroleum and hydrocarbon gases is to use perforating guns wherein the explosive trains (i.e. the detonator, detonating cord and shaped charges) are exposed to wellbore fluids. It is possible in hot deep wells to encounter temperatures in excess of 500.degree. F. and pressures over 20,000 psi. Thus, considerable stress can be placed on the explosive train, particularly the detonating cord since it is encased in a pressure transmitting jacket which subjects the explosive in the cord to the simultaneous influence of temperature and pressure. Explosives decompose at some finite rate under the influence of heat. They tend to decompose even faster when both high pressure and high temperature are present. Because of the nature of its function in high temperature wellbores, it is necessary to manufacture detonating cords from thermally stable explosive compounds. Two such explosives which are commonly used for this purpose are known as PYX and ONT. Both of these thermally stable detonating compounds possess excellent thermal stability, up to 500.degree. F. for 100 hours, and are relatively easy and inexpensive to manufacture.
Unfortunately, however, detonating cords made from these explosives can be difficult to initiate or start in the explosive process. Three factors which can contribute to the difficulty in initiating these compounds are:
(1) the inherent thermal stability of many high temperature explosives makes them insensitive, not only to heat, but usually to other initiation stimuli such as shock or impact;
(2) downhole pressure acting on the detonating cord increases its density which tends to decrease the explosive sensitivity of the cord; and
(3) the explosive particle size that is best for detonating cord manufacture is usually not conducive to initiation.
Thus detonating cords which contain thermally stable explosives under the influence of downhole wellbore pressure can be extremely difficult to initiate. Conventional detonators simply might not generate sufficient shock strength to initiate these detonating cords.
A detonator package for use in this type of environment is described in U.S. Pat. No. 4,759,291 which is assigned to the common Assignee of the present application. This patent and its disclosure are incorporated herein by reference. Even utilizing a detonator package such as that described in the foregoing patent one may encounter difficulty in initiating high temperature cords. However, it may be possible to improve the functioning of a detonator device such as that shown in the aforementioned U.S. patent by using special detonation transfer techniques and apparatus. Such techniques and apparatus are the subject of the present application.